Since the introduction of X-ray computed tomography (“CT”) in 1972, it has become apparent that low-energy photons are absorbed by the body more readily than high-energy photons. This differential in absorption is known in the art as the “hard-beam effect,” and presents several limitations. The implementation of spectral CT has become an area of focus in counteracting the hard-beam effect.
Various hardware systems have been developed to implement spectral CT. For example, in dual-energy CT, a CT machine (such as a GE Discovery CT750 HD) scans an object with two distinct energy spectra. Dual-energy CT machines are rare and costly. Another strategy is to use a single X-ray spectrum but with energy-discriminative detectors to capture responses at different energies, and the data is processed using spectral CT iterative reconstruction algorithms. Others have attempted to improve the hardware of CT machines to reduce hard-beam artifacts, through methods such as pre-filtering the X-ray beam.
Several software approaches have also been developed. Early attempts sought to reduce beam-hardening streak artifacts, and the idea of spectral CT emerged. An iterative maximum-likelihood polychromatic algorithm for CT (“IMPACT”) was developed. IMPACT seeks to find the attenuation coefficient distribution that maximizes the log-likelihood function. It was demonstrated that the IMPACT algorithm is good at eliminating beam-hardening artifacts, and it can be applied in reducing metal artifacts. The use of a filtered back projection (“FBP”) reconstruction algorithms has been the most popular method of reconstructing CT images. In the FBP reconstruction algorithm, a linear attenuation coefficient is reconstructed. In these applications, it is assumed that the linear attenuation coefficient is energy-independent.
However, as discussed above, when X-rays penetrate the human body, low-energy photons are more readily absorbed than high-energy photons. In particular, linear attenuation coefficients reconstructed by the FBP algorithm tend to have a poor contrast resolution for low-density soft tissues and therefore yield poor results in the diagnosis of challenging lesions. Many iterative FBP reconstruction algorithms have also been introduced since the first commercial CT scanner was developed in 1972. Typically, these prior art iterative algorithms also suffer from the hard-beam effect because the spectrum of X-rays is ignored in computing the underlying attenuation coefficients.
Finally, many practitioners utilizing FBP and other reconstruction algorithms inject iodine contrast material into the blood to address the hard-beam effect. However, an iodine injection can cause medical complications and increase costs, and therefore does not present an ideal spectral CT solution.
There is a need in the art for improved systems and methods for the implementation of spectral CT. Unlike the hardware improvements discussed above, the disclosed embodiments can realize spectral CT by implementing iterative algorithms on existing CT hardware platforms.